The present invention relates to antennas and more particularly to a novel whip antenna configuration providing uniform electrical performance over a broad band of frequencies.
Vehicles equipped for radio communications require specialized antenna installations. The antenna height is restricted by obstacles, such as highway overpasses and tunnels, under which the vehicle must travel without damaging the antenna. The antenna must exhibit a large degree of resiliency to avoid wind-load distortion or breakage when the vehicle is in motion and yet return as quickly as possible to a vertical direction for minimum radiation polarization loss when the wind-load diminishes. The antenna must possess a uniform circular radiation pattern in the horizontal plane so that radio communications can be established without regard to the vehicle's direction of travel.
One antenna type which exhibits these characteristics is the whip antenna, which is characterized by a long, thin conductive cylindrical member attached to an insulated mounting base. The radio communications equipment is electrically connected by a cable to the base end of the whip. The electrical length of the basic whip is usually chosen to be less than a full wavelength. In an antenna one-half wavelength long, for example, the antenna impedance approaches the impedance of commonly used coaxial cables and an acceptable VSWR is obtained. Most of the transmitter output power is dissipated by the antenna and is not reflected back to the transmitter. The antenna impedance is extremely frequency sensitive due to the shifting of the minimae of the standing wave of current along a fixed length antenna. As the operating frequency varies over the frequency range of operation, the impedance changes and the antenna and cable impedance are no longer closely matched. The VSWR and the amount of reflected power both increase causing a decrease in the amount of radiated power and antenna efficiency. Because the whip antenna is only efficient over a small range of frequencies, it is not uncommon for vehicles with multiple frequency transmission capability to require several different length antennas. A whip antenna having a constant impedance over a broad band of radio frequencies is desirable.